Germline stem cells reside in the reproductive organs, i.e., the ovaries and testes, represent potentially one of the most important and protected classes of stem cells in the mammalian body. Genetic conservation and high telomerase activity has been reported in stem cells derived from these tissues, as well as, extensive DNA modification with chromatin chromosomal modifications. Scientists have differed about what types of stem cells are resident in adult reproductive tissues, as well as, their potentiality in differentiation.
Chemotherapy and radiation treatments not only target cancerous cells, but also rapidly dividing cells. In the testes, the rapidly dividing germ cells are highly sensitive to these exposures. In the prepubertal testis, germline stem cells are similarly sensitive and are acutely and dose-dependently depleted following radiation exposure. Low doses of cytotoxic drugs or irradiation deplete the differentiating spermatogonia while less sensitive spermatogonial stem cells as well as spermatocytes and spermatids may survive. The differentiating germ cells can continue their maturation into sperm cells and can re-colonize the seminiferous tubules with stem cells which are generated from the surviving stem cell population.
However, in cases of severe depletion, spermatogeneis may only be restored in a very few seminiferous tubules, thereby limiting fertility. Patients will be permanently infertile after complete depletion of testicular stem cells. The impact on spermatogenesis manifests itself most acutely at, or before, the time of puberty because sperm cannot be typically obtained and cryopreserved as in postpubertal males. Even a few stem cells can re-colonize the seminiferous tubules if given sufficient time to re-initiate spermatogenesis.
Until recently, it was believed that female gonads of most mammalian species, including humans, house a finite number of meiotically-arrested germ cells (oocytes) enclosed within primordial follicles that serve as the stockpile of eggs released at ovulation during each menstrual cycle. Oocyte numbers decline throughout postnatal life, through mechanisms involving apoptosis, which were widely believed to eventually leave the ovaries barren of germ cells. In humans, exhaustion of the oocyte reserve typically occurs during the fifth decade of life, driving menopause.
According to this basic doctrine of reproductive biology, it was further believed that once depleted, the ovarian germ cell pool could not be replenished. Thus, any treatment that accelerates the loss of oocytes threatens to decrease the fertility and will cause menopause at an earlier age than expected. For example, exposure of women to a wide spectrum of agents that damage the ovary, such as chemotherapeutic agents and radiotherapy, generally leads to premature menopause and irreversible sterility. At present, the limited therapeutic options of preserving fertility and normal ovarian function under various adverse conditions are invasive, such as, for example, cryopreservation of ovarian tissue fragments or single oocytes, and often require prior hormonal therapy, which can be medically inappropriate for many women with hormonally responsive tumors. In addition, there are currently no therapeutic options for postponing normal ovarian failure at menopause.
Two primary approaches have been identified for restoration of functional germ cells in both males and females. The first is the grafting of immature tissue (either ovarian or testis) tissue fragments onto the surviving tissue and the second is based on isolation and transplantation of stem cells.
Germ cell transplantation has been developed in rodent animal models. Microinjection of germ cells from mice or closely-related species into the seminiferous tubules of a mouse re-stimulated spermatogenesis from donor spermatogonial stem cells. The spermatogonial cells can be cryopreserved or cultured prior to transfer. Similarly cyropreserved ovarian cortical tissue has been transplanted in sheep and human with the resulting resumption of estrus and the birth of live offspring following normal matings. However, such direct autologous transplantation back into the donor organism may not be optimal for patients who were originally treated for malignancies. This is because if such patients relapse after chemotherapy or radiation therapy, it is not clear whether the relapse could be from reservoirs of malignant cells in the body that were not killed with chemotherapy or radiation therapy or if the re-transplantation of cryopreserved gonadal tissue and cells harbored such malignant cells. As such, the optimal approach for such patients is to mature the cryopreserved gonadal tissue into functional sperm or eggs, and perform in vitro fertilization (with or without intracytoplasmic sperm injection or ICSI) with the natural or similarly ex host matured egg or sperm of the partner, and then after re-implant the fertilized embryo into female partner. This approach would be free from any possibility of tumor contamination as only the fertilized embryo formed from uncontaminated ex host matured sperm or egg.
Therefore, a germline cell banking system is needed in humans to restore reproductive potential in patients without the ability to bank mature sperm or ova and to mature the banked germline cells ex host/ex vivo or ex host/in vivo to produce mature sperm or ova.